The present invention relates to improved low friction oil compositions using lubrication additives and to methods for improving friction reduction in employing lubricating oils prepared therefrom. More specifically, the invention relates to a friction modifier additive containing a combination of organomolybdenum compounds which demonstrate a synergistic combination as a friction modifier in lubricating oils.
Motor vehicle manufacturers have sought to improve fuel economy through engine design but also through designing engines which take advantage of new performance oils which have better fuel efficiency, oxidative stability, volatility, and improved viscosity index to name a few characteristics over conventional formulations. Engine oils have played an important role in improving fuel economy and resulting improved emission characteristics of motor vehicles, due to their low cost per unit in fuel efficiency in comparison with engine hardware changes. To reduce friction and improve fuel efficiency, there has been a drive to use lower viscosity engine oils, which often requires new additive package formulations. High on the list of requirements for these new formulated engine oil specifications are those employing friction modifiers in the lubricating oil composition. In this case, the additive system design is the crucial factor playing close attention to the additive/additive and additive/base fluid interactions.
Engine oil acts as a lubricant between moving engine parts at various conditions of load, speed and temperature. Hence, the various engine components experience different combinations of boundary layer, mixed and (elasto) hydrodynamic regimes of lubrication; with the largest frictional losses at piston liner/piston ring interface and a smaller part by the bearing and valve train. To reduce the energy losses due to friction of the various parts and to prevent engine wear, additives are incorporated into the engine oil such as friction modifiers, anti-wear agents, antioxidants, dispersants, detergents, etc. Also to reduce the hydrodynamic friction in the piston/cylinder the viscosity of engine oils has been lowered which has increased the dependence of friction modifiers to offset the new boundary layer regime.
Friction modifiers have been around for several years for application in limited slip gear oils, automatic transmission fluids, slideway lubricants and multipurpose tractor fluids. With the desire for increased fuel economy, friction modifiers have been added to automotive crankcase lubricants and several are known in the art. Well known friction modifiers can be classified into different groups regarding their function. Mechanically working friction modifiers are generally used in solid lubricating compounds, e.g. molybdenum disulfide, graphite, PTFE, polyamide; adsorption layer forming friction modifiers include, for example, higher fatty acids, e.g. oleic acid and stearic acid; higher alcohols, e.g. oleyl alcohol; esters; amines; sulfide oils. Friction polymer forming friction modifiers are generally ethoxylated dicarboxyclic acid partial esters, dialkyl phtalic acid esters, methacrylates, unsaturated fatty acids, and the remaining class is referred to as organometallic compounds represented by copper containing organic compounds and molybdenum compounds such as molybdenum dithiophosphates, molybdenum dithiocarbamates.
Friction modifiers generally operate at boundary layer conditions at temperatures where anti-wear and extreme pressure additives are not yet reactive by forming a thin mono-molecular layers of physically adsorbed polar oil-soluble products or reaction layers which exhibit a significantly lower friction compared to typical anti-wear or extreme pressure agents. However, under more severe condition and in mixed lubrication regime these friction modifiers are added with an anti-wear or extreme pressure agent. The most common type is a zinc dithiophosphate (ZnDTP) or zinc dithiocarbamate (ZnDTC).
However, when friction modifiers are added with other polar additives which also have an affinity to metal surfaces such as anti-wear, extreme pressure, anti-corrosion as well as detergents and dispersants, the friction modifier can compete for the active surface site or interact with each other. For example, anti-wear agents such as ZnDTP and ZnDTC protect closely approaching metal surfaces from asperities from damaging the opposite surface. These films are semi-plastic which are difficult to shear off so that under shearing conditions, their coefficient of friction is generally high. Conversely, a friction modifier generally operates by building an orderly and closely packed arrays of multi-molecule layers which are attracted to the metal surface via their polar heads and aligned to each other via Van der Waal forces. Therefore, when surface active agents such as anti-wear agents ZnDTP, a friction modifier or a detergent are added to a lubricating oil, the adsorption of the anti-wear agent is reduced by the competitive adsorption of the other agents. Accordingly, the selection of components and interactions between them is of major concern and synergistic interactions are not expected or possible to anticipate. Thus synergies when discovered, especially when found at concentrations of additives not employed or useful for that purpose in the art, further the advances and new requirements for formulating new oil compositions.
Molybdenum compounds are known in the art to be useful as antioxidants, friction modifiers and to provide anti-wear and extreme pressure resistance properties in lubricating oils. For example:
U.S. Pat. Nos. 4,259,194; 4,259,195; and 4,261,843 disclose antioxidant additives for lubricating oil that are prepared by combining a polar promoter, an acidic molybdenum compound, and certain basic nitrogen compounds to form a molybdenum-containing composition.
U.S. Pat. No. 4,265,773 discloses antioxidant additives for lubricating oil that are prepared by combining an acidic molybdenum compound, an oil-soluble basic nitrogen compound, and carbon disulfide to form a sulfurxe2x80x94 and molybdenum-containing composition.
U.S. Pat. Nos. 4,263,152 and 4,272,387 discloses antioxidant additives for lubricating oil that are prepared by combining an acidic molybdenum compound, a basic nitrogen compound complex, and a sulfur source to form a sulfurxe2x80x94 and molybdenum-containing composition.
U.S. Pat. No. 4,283,295 discloses antioxidant additives for lubricating oil that are prepared by combining a polar promoter, ammonium tetrathiomolybdate, and a basic nitrogen compound complex to form a sulfurxe2x80x94 and molybdenum-containing composition.
U.S. Pat. No. 4,285,822 discloses antioxidant additives for lubricating oil that are prepared by (1) combining a polar solvent, an acidic molybdenum compound, and an oil-soluble basic nitrogen compound to form a molybdenum-containing complex and (2) contacting said complex with carbon disulfide to form a sulfurxe2x80x94and molybdenumxe2x80x94containing composition.
U.S. Pat. No. 4,315,826 discloses multipurpose lubricant additives that are prepared by reaction of carbon disulfide with thiomolybdenum derivatives of polyalkenylsuccinimides having basic nitrogen functions. It is said that the subject additives function as dispersants possessing excellent antifriction properties and impart anti-wear and antioxidant properties to a lubricant.
U.S. Pat. No. 4,369,119 discloses antioxidant additives for lubricating oil that are prepared by combining (a) a sulfur-containing molybdenum compound prepared by reacting an acidic molybdenum compound, a basic nitrogen compound, and a sulfur compound, with (b) an organic sulfur compound.
U.S. Pat. No. 4,395,343 discloses antioxidant additives for lubricating oil that are prepared by combining (a) a sulfur containing molybdenum compound prepared by reacting an acidic molybdenum compound, a basic nitrogen compound, and carbon disulfide, with (b) an organic sulfur compound.
U.S. Pat. No. 4,402,840 discloses antioxidant additives for lubricating oil that are prepared by combining (a) a sulfur containing molybdenum compound prepared by reacting an ammonium thiomolybdate compound, and a basic nitrogen compound, with (b) an organic sulfur compound.
U.S. Pat. No. 4,474,673 discloses antifriction additives for lubricating oil that are prepared by reacting a sulfurized organic compound having an active hydrogen or potentially active hydrogen with a molybdenum halide.
U.S. Pat. No. 4,479,883 discloses a lubricating oil composition that is said to have particularly improved friction reducing properties that comprises an ester of a polycarboxylic acid with a glycol or glycerol and a selected metal dithiocarbamate and that contains a relatively low level of phosphorus.
U.S. Pat. No. 4,501,678 discloses a lubricant containing molybdenum dialkyldithiocarbamates that is said to be useful for improving the fatigue life of gears.
It is well known in the art that formulating engine oils there is a competitive adsorption between friction modifiers and other surface active agents. U.S. Pat. Nos. 5,672,572 and 5,814,587 disclose that anti-wear agents such as ZDDP compete with organomolybdenum compounds for the metal surface.
This invention is directed to the unexpected synergy and resulting low friction coefficient in lubricating compositions containing a major amount of an oil of lubricating viscosity and at least 450 parts per million of molybdenum based upon the total mass of the composition of a friction modifier containing an unsulfurized and/or sulfurized oxymolybdenum nitrogen dispersant complex and a sulfurized oxymolybdenum dithiocarbamate employed at a low concentration.
The unsulfurized or sulfurized oxymolybdenum containing composition can be prepared by (i) reacting an acidic molybdenum compound and a basic nitrogen compound selected from the dispersant group consisting of succinimide, a carboxylic acid amide, a hydrocarbyl monoamine, a phosphoramide, a thiophosphoramide, a Mannich base, a dispersant viscosity index improver, or a mixture thereof in the presence of a polar promoter, to form an oxymolybdenum complex. This oxymolybdenum complex can be reacted with a sulfur containing compound, to thereby form a sulfurized oxymolybdenum containing composition, useful within the context of this invention. Preferably the dispersant is a polyisobutenyl succinimide. The oxymolybdenum or sulfurized oxymolybdenum containing compositions may be generally characterized as a sulfur/molybdenum complex of a basic nitrogen dispersant compound preferably with a sulfur to molybdenum weight ratio of about (0.01 to 1.0) to 1 and more preferably from about (0.05 to 0.5) to 1 and a nitrogen to molybdenum weight ratio of about (1 to 10) to 1 and more preferably from (2 to 5) to 1. The precise molecular formula of these oxymolybdenum compositions are not known with certainty. However, they are believed to be compounds in which molybdenum, whose valences are satisfied with atoms of oxygen or sulfur, is either complexed by, or the salt of one or more nitrogen atoms of the basic nitrogen atoms of the basic nitrogen containing compound used in the preparation of these compositions. In one aspect, the oxymolybdenum complex is prepared at a reaction temperature at or below 120 degrees centigrade and if optionally sulfurized, it is also reacted at or below 120 degrees centigrade. Such a process yields a lighter color product when compared to higher temperature reaction conditions at equivalent pressure.
In addition to the oxymolybdenum nitrogen containing dispersant described above, the present invention includes a small amount of a molybdenum dithiocarbamate of the formula I 
wherein R1, R2, R3 and R4, are independently selected from a hydrocarbon group; X1 to X4 are independently selected from sulfur or oxygen atom; wherein said molybdenum dithiocarbamate is present below 175 ppm in terms of molybdenum concentration, based upon the total mass of the lubricant composition. In a preferred aspect, the molybdenum dithiocarbamate is present from 10 to 175, more preferably 25 to 150, also preferred below 100 and from 50 to 90, all in terms of ppm of molybdenum concentration of the molybdenum dithiocarbamate, based upon the total mass of the composition.
Lubricating oils comprising a major amount of an oil of lubricating viscosity with a) an oxymolybdenum nitrogen containing dispersant and b) a molybdenum dithiocarbamate can be employed at a ratio of a) to b) from 2:1 to 20:1 and preferably from 5:1 to 10:1. Additionally, such compositions can further comprise a detergent, preferably a calcium phenate and/or an ashless dithiocarbamate.
The compositions exhibit a synergistic reduction in the measured friction coefficient and accordingly are useful for reducing the friction characteristics when employed in a lubricating oil. Therefore, another aspect is directed to uses and to methods for improving the friction reduction performance in lubricating oil by adding an effective amount of an oil soluble or dispersible amount to the friction modifier composition described herein.